The inventors have observed that nitridation of 3D device structures cannot be easily performed using typical plasma ion exposure due to the non-conformal nature of the plasma sheath, which prevents conformal doping of the top surface of a film and the device sidewall. Instead, the inventors believe that 3D conformal nitridation requires radical or neutral species driven reactions. One method of nitridizing a hafnium oxide based 3D high-k gate stack is through the use of an inductively coupled plasma generated using ammonia and, optionally, an inert gas, and/or nitrogen gas (N2). However, the inventors have observed that this process also leads to the formation of a number of reactive hydrogen species, including both hydrogen radicals and hydrogen ions. These reactive hydrogen species can potentially penetrate the nitridized film and negatively interact with the gate stack materials. Additionally, the inventors have observed that this process also leads to the formation of a number of inert gas and/or nitrogen ions, which also undesirably contribute to the non-conformal processing results. The inventors propose that reducing or eliminating the reactive hydrogen species prior to their penetration and interaction with the gate stack materials can prevent device failure, and reducing or eliminating the inert gas and/or ions prior to their interaction with the substrate can enhance conformal processing results.
As such, the inventors have provided improved methods and apparatus for nitridizing materials, such as those in 3D device structures.